The problem with alcohol is , it does not freeze by itself., so I would recommend to mix your chocolate and kahlua together in the double boiler then proceed as you would normally with this recipe, doing it this way you should have no problem with them setting up.This is a old trick that was taught to me while working for fanny farmer candies.
I am not rating these just adding comments. These were a very difficult truffle for me to do. I left them in the freezer for 2 days and they didn't harden up at all. They were still as gooey as when I put them in. They were impossible to roll and impossible to dip. I ended up coating mini muffin pans with chocolate and dropping spoonfuls of the truffle mix into them and topping them with more chocolate. After they were completely covered with chocolate and the chocoalte hardened, they were fine. they tasted good but were a nightmare to make. I don't know what I did wrong, but they would not firm up at all.